Agricultural and Forest Meteorology 104 2000 119–131
Theoretical relationship between stomatal resistance and surface temperatures in sparse vegetation
J.P. Lhomme
∗ ,
1
, B. Monteny
Centre d’Ecologie Fonctionnelle et Evolutive, CNRS, 1919 route de Mende, 34293 Montpellier Cedex 5, France Received November 1999; received in revised form 5 April 2000; accepted 12 April 2000
Abstract
The relationship between stomatal resistance and foliage temperature in sparse vegetation has been the subject of previous papers [Smith, R.C.G., Barrs, H.D., Fischer, R.A., 1988. Agric. Forest. Meteorol. 42, 183–198; Shuttleworth, W.J., Gurney,
R.J., 1990. Q. J. R. Meteorol. Soc. 116, 497–519], in which the modeling is based upon the one-dimensional two-layer approach of Shuttleworth and Wallace [Shuttleworth, W.J., Wallace, J.S., 1985. Q. J. R. Meteorol. Soc. 111, 839–855]. In both
studies, however, a major assumption exists concerning the contribution of the substrate to the evaporation process. Using the same approach as these previous studies, an extended and upgraded model is presented in the sense that it relates stomatal
resistance to foliage and substrate temperatures T
f
and T
s
without any assumption on substrate contribution. A comparison of stomatal resistances estimated from component temperatures T
f
and T
s
with values measured on fallow savannah during the HAPEX–Sahel experiment confirms the good performance of the model. Numerical simulations show the general behavior of
the relationship between stomatal resistance and foliage temperature in several scenarios involving various weather conditions and canopy characteristics. The sensitivity of the calculated stomatal resistance to input variables and model parameters is
investigated. It is shown that the calculation of stomatal resistance exhibits a significant sensitivity to foliage temperature and a much lesser one to substrate temperature. Uncertainties in leaf area index have a relatively weak impact on the calculated
stomatal resistance. The sensitivity of stomatal resistance to the two main coefficients involved in the partitioning of available energy has also been investigated. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Sparse vegetation; Stomatal resistance; Two-layer model; Radiometric surface temperature; Foliage and substrate temperature
1. Introduction
In the assessment of vegetation water status the measurement of foliage temperature by infrared
thermometry can be very helpful as a stress indicator. Its use stems from the fact that when the stomatal
closure resulting from a water stress occurs, the tran-
∗
Corresponding author. Fax: +33-4-67-4121-38. E-mail address:
lhommecefe.cnrs-mop.fr J.P. Lhomme
1
Permanent address: IRD previously ORSTOM, 213 rue La Fayette, 75010 Paris, France.
spiration rate reduces and foliage temperature rises due to a decrease in energy dissipation. The idea of
using plant foliage temperatures as a stress indicator has been popularized in the early 1980s by Idso and
Jackson Idso et al., 1981; Jackson et al., 1981 in the form of the crop water stress index CWSI. This
index, based upon the difference between the actual canopy temperature and that of an unstressed base-
line, has led to important agricultural applications such as irrigation scheduling and crop yield predic-
tion. When vegetation is sparse and when temperature measurements are made from hand-held, airborne or
0168-192300 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 9 2 3 0 0 0 0 1 5 5 - 6
120 J.P. Lhomme, B. Monteny Agricultural and Forest Meteorology 104 2000 119–131
satellite-based infrared sensors, foliage temperature is often difficult to separate from the soil component,
which makes the CWSI poorly reliable. To overcome this difficulty, Moran et al. 1994 developed the veg-
etation index-temperature VIT trapezoid method, based on a combination of spectral vegetation indices
and composite surface temperature measurements. Jones 1999 modified the calculation of the CWSI
by replacing theoretical estimates of the upper bound and baseline temperatures by measured temperatures
of appropriate reference surfaces.
Concurrently, attempts
have been
made to
directly determine canopy resistance from the surface temperature of the foliage, as measured by infrared
thermometry, in conjunction with standard meteoro- logical measurements above the canopy. When there
is complete canopy cover, the single-layer approach underlying the Penman–Monteith equation Monteith,
1965 provides an adequate basis for this determina- tion Jackson et al., 1981; Hatfield, 1985. For sparse
vegetation, the problem is not so straightforward. Smith et al. 1988 developed and tested a two-layer
approach based on the Shuttleworth–Wallace model 1985 to calculate the mean stomatal resistance of
a sparse crop from infrared measurements of foliage temperature. However, a major drawback of their
approach is that the theoretical expression of foliage resistance is a function of soil evaporation that is a pri-
ori unknown. Their assumption of using equilibrium evaporation as an estimate of soil evaporation limits
the applicability of their method because evidently it does not hold when soil dries out. In a similar man-
ner, Shuttleworth and Gurney 1990 investigated the performance of the Shuttleworth–Wallace model to
calculate the canopy resistance from measurements of foliage temperature. But the exacting assumption ap-
pearing in Smith et al. 1988 still exists in the form of a fixed and arbitrary value of soil surface resistance.
They recognize quote that: “In practical applica- tions a better specification of substrate interaction is
identified as the primary problem in using this theory in very sparse canopies”, adding that “a measurement
of substrate temperature used in conjunction with [basic equations] represents an attractive and simple
alternative” Shuttleworth and Gurney, 1990, p. 517.
The present study follows the lines drawn by these previous papers in the sense that it aims at present-
ing and evaluating the theoretical basis needed to infer stomatal resistance in sparse vegetation from measure-
ments of radiometric surface temperature. It is shown that stomatal resistance can be calculated in a more re-
liable manner than in the previous models, i.e. without major assumption concerning the substrate interaction,
when foliage and substrate temperatures are simul- taneously used in the calculation. Consequently, the
model presented constitutes an upgrading of the pre- vious models of Smith et al. 1988 and Shuttleworth
and Gurney 1990. The paper is divided into two main sections. The first section infers the theoretical expres-
sion relating stomatal resistance to foliage and sub- strate temperatures. The second section presents the
practical implementation of the model with an experi- mental validation, numerical simulations and a sensi-
tivity analysis.
2. Theoretical development
